54 CHEMICAL ENGINEERING
The rotary transformer is used to change alternating to direct current when both are of about the same voltage. One set of windings answers for the alternating-current motor and the direct-current generator. Full-load efficiency, about 0.93. Where the voltages differ greatly, the more expensive motor generator set must be used (efficiency about 0.85). The two machines then have only the shaft and bearings in common. Such a unit may be used to change frequency if desired: parallel operation then presents some difficulties. A rotary condenser is a synchronous motor introduced at some point where there is an easily started load to improve the power factor of a system largely made up of induction
Storage batteries are used for direct-current stand-by service, improvement of regulation, excitation of alternators, etc. Cells are in series and in sufficient number to give the required voltage. Charging is by direct current. Ventilation is necessary for removal of gases from the battery room. There are two types: the alkaline (Edison) using nickel hydroxide and iron plates in a potassium hy^ droxide electrolyte: and the lead, with plates of lead and lead peroxide in dilute sulphuric acid. Capacities vary, and are expressed in ampere-hours, based on an assumed time of discharge (8 hr. for lead, 5 hr. for alkaline). Efficiencies at such normal rate are around 0.70. During charging, the voltage is increased (1.7 to 1.95 for alkaline, 2 to 2.8 for lead, per cell). During discharge, the voltage falls: 1.54 to 0.88 for alkaline, 2.0 to 1.75 for lead. Costs are very high: in normal times, from $100 to $200 per kilowatt developed when discharging in 1 hr. This is three or four times the capacity realized at the normal discharge rate.
Heating Processes.—Electric furnaces are of the arc, induction or resistance type. The last are used for warming of rooms, requiring 1 to 2 watts per cubic foot of room contents at zero outdoor temperature. The resistance element must be non-oxidizable if it is exposed to the air. Electric house warming is too costly for any but the most exceptional use. In industrial furnaces, electricity gives a high temperature (up to 4,000°C.), localized and susceptible of close regulation. "The energy is-employed to heat up the charge, sometimes to melt or vaporize it or to produce chemical reaction, and (wastefully) to cover conduction and flue gas losses and electrical (resistance) losses external to the furnace. Furnace linings must be refractory (carbon), but these should be backed by walls of non-conducting material. Arc furnaces are built for capacities up to 3,000 kw. per pair of electrodes (three-phase current) for operation at either atmospheric or higher pressure. Induction furnaces are virtually immense static transformers, the low-tension windings of which are replaced by baths of molten metal, for example. The amount of current is limited by the so-called "pinch effect." In resistance furnaces, the charge itself may constitute the resistance element.
Electroplating, Etc.—The energy required for electroplating, galvanoplasty, detinning and electrolytic refining of metals varies with the metals involved and solutions used. From 1 to 100 amp. may be needed per square inch of cathode surface, at 0.1 to 4 volts per cell. Direct current is supplied from small generators at 5 or 6 volts, and a separate rheostat is required for each cell or tank.
Distribution of Electricity.—Sizes of wires must be suitable for maximum current from the standpoint of heating effect (see table) and must generally be based on the corresponding drop of voltage. For primary mains and feeders a